1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image which is adapted for use in electrophotography or the like; a process cartridge housing such a toner; an image forming apparatus used with such a toner; an image foxing process using such a toner; and an image forming process using such a toner.
2. Description of the Related Art
Various electrophotographic processes have been proposed including those described in U.S. Pat. No. 2,297,691, Japanese Patent Application Publication (JP-B) Nos. 49-23910 and 43-24748. An electrophotography, however, is generally a process accomplished by forming a latent electrostatic image on a photoconductor by various means utilizing the photoconductor material, developing the latent image by using dry toner, transferring the toner image to a paper or the like, and fixing the transferred image by applying heat, pressure, or the like to thereby produce a photocopy.
The processes used for developing the latent electrostatic image can be roughly categorized into a liquid development process using a developer wherein various pigments and dies are finely dispersed in an insulating organic liquid, and a dry development process such as cascade development, magnetic brush development, and powder cloud development wherein a toner having carbon black or other colorants dispersed in a natural or synthetic resin is used. The dry development process includes a single component development process and a two component development process wherein a carrier is used together with the toner.
The process used for thermally fixing the toner image onto the transfer paper can be generally categorized into contact fixing and non-contact fixing methods. The contact fixing method includes those using a heat roller and those using a belt. The non-contact fixing method includes a flash fixing and an oven (atmospheric) fixing.
The fixing method using a heat roller is highly effective since the toner image and the heat roller are brought into direct contact with each other, and this enables the production of compact apparatus. Accordingly, this method is widely employed.
However, because of the recent requirement for higher energy efficiency, a heat source is often turned off during the waiting period when no fixing is conducted. In such apparatus, heating of the heat roller should start as soon as the heat source is turned on and the predetermined desirable temperature should be attained in a very short period.
In the case of the fixing unit used in such apparatus, attempts have been made to reduce the thickness of the fixing roller that becomes in contact with the toner image-bearing surface to the level of 1.0 mm or less to thereby improve the thermal energy efficiency (Japanese Patent Application Laid-Open (JP-A) Nos. 2002-82474 and 09-222750). Such attempts have paid off, and the time required to attain the predetermined desirable temperature has been reduced to a very short period.
Reduction of the thickness of the fixing roller to the level of 1.0 mm or less, however, resulted in the loss of the mechanical strength of the roller itself. A large load cannot be applied to the roller because the roller is deformed by such a large load.
This situation requires a toner adapted for use of such a fixing unit having a low temperature fixability of the level incomparable to the conventional toners.
Various technical proposals have been made to solve such situation.
For example, Japanese Patent (JP-B) No. 2743476 proposes a toner adapted for use in roller fixing, wherein the toner comprises a core particle consisting of a polyester resin and a wax having a polar group and the core particle is coated with a resin, and in this patent, the polyester resin and the wax are defined for their melt viscosity. JP-A Nos. 03-122661, 04-85550, and JP-B No. 08-16804 disclose toners to be fixed on a film, comprising a particular polyester and a particular release agent, and in these applications, the melt viscosity of the polyester resin at 80° C. to 120° C., the gradient of the curve of the melt viscosity in relation to the temperature, and the melt viscosity of the release agent are defined. JP-B No. 08-12459 proposes a capsule toner to be fixed on a film, comprising a particular polyester resin and a particular release agent, and in this application, the melt viscosity of the polyester resin at 80° C. to 120° C., the gradient of the curve of the melt viscosity in relation to the temperature, and the melt viscosity of the release agent are defined. JP-B No. 07-82250 discloses a toner be fixed on a film, comprising a particular polyester resin, a particular organometallic compound, and a particular release agent, and in this application, the melt viscosity of the polyester resin at 120° C. to 150° C., the gradient of the curve of the melt viscosity in relation to the temperature, and the melt viscosity of the release agent are defined. JP-B No. 07-72809 proposes a toner comprising a styrene-acryl resin, and in this application, the relation between the melt viscosity measured at 110° C. to 130° C. and the temperature are defined. JP-A No. 10-246989 proposes a toner comprising a particular charge control agent, and in this application, the average viscosity gradient of the toner is defined.
In addition, corresponding to the demand for the reproduction of a high quality image, there has been a trend towards the toner particle size to be reduced. A toner with a smaller particle size is known to have a poor fixability since it becomes difficult to apply a sufficient pressure to such small toner particles between fixing rollers. This trend is even more significant in the fixing unit operated at a low bearing force.
Attempts have also been made to replace carbon black with other colorants, for example, fine powders of black metal compounds with high safety and good thermal conductivity.
For example, JP-B No. 2736680 proposes a mixture of Fe2TiO5 and Fe2O3-FeTiO3 solid solution having an average particle size of 0.1 μm to 0.5 μm; JP-B Nos. 3101782, 3108823, and 3174960 disclose a magnetic iron oxide comprising 25% to 30% of FeO; JP-B Nos. 3224774 and 3261088 propose a magnetite having a residual magnetization of 6 emu/g or less; JP-A No. 2000-319021 discloses iron oxide particles wherein the interior comprises Ti, and the surface comprises Ti and Fe; JP-A No. 2002-129063 discloses a rutile TiO2 mixed phase crystal having a saturation magnetization of 0.5 emu/g to 10 emu/g and a particle size of 0.1 μm to 0.4 μm; JP-A No. 2002-189313 proposes a metal compound having a saturation magnetization of 30 emu/g or less and a dielectric loss of 50 or less; and JP-A No. 2002-196528 discloses a metal compound having a saturation magnetization of 40 emu/g or less used at a content of 20 parts by weight or less; and JP-A No. 2000-10344 discloses black hematite fine particles and black hydrous iron oxide fine particles.
However, when the fixing unit having a roller with reduced thickness operated at a low bearing force is used to correspond to the increasing demand of energy saving, toners produced by the prior art technologies are far from being sufficient to realize the required fixability.
In addition, the colorants used as substitutes for the carbon black have been insufficient in electroconductivity, and the charge build up speed and stability thereof have been insufficient.